The invention relates to adaptive channel equalizers for digital television receivers and, more particularly, to overcoming the effects of co-channel NTSC interference upon the updating of filter coefficients for such equalizers.
The updating of filter coefficients for an adaptive channel equalizer included within a digital television (DTV) receiver is accomplished by one of two known types of adaptation method or by a combination of these two adaptation method types. The first type of adaptation method is referred to as the training signal type. The second type of adaptation method is referred to as the decision-directed type. Both types of adaptation method can be disrupted by demodulation artifacts of NTSC co-channel interference accompanying the demodulated baseband symbol coding recovered by the DTV receiver.
U.S. Pat. No. 5,331,416 entitled xe2x80x9cMETHODS FOR OPERATING GHOSTxe2x80x94CANCELATION CIRCUITRY FOR TV RECEIVER OR VIDEO RECORDER issued Jul. 19, 1994 to C. B. Patel and J. Yang is of interest in regard to the training signal type of channel equalizer adaptation. U.S. Pat. No. 5,331,416 indicates that training signal components accumulate in accordance with scalar alegebra to exhibit increase in value relative to random noise, which accumulates in accordance with vector algebra to lower value.
A method to reduce the effect of demodulation artifacts of NTSC co-channel interference upon the adaptation method of the training signal type is described in U.S. patent application Ser. No. 09/151,209 filed Sep. 11, 1998 for A. L. R. Limberg, titled xe2x80x9cMETHOD OF OPERATING THE CHANNEL EQUALIZER IN A RECEIVER FOR DTV SIGNALS SUBJECT TO CO-CHANNEL NTSC INTERFERENCExe2x80x9d, and assigned to Samsung Electronics Co., Ltd. pursuant to the terms of an employment agreement in force at the time the invention was made. U.S. patent application Ser. No. 09/151,209 describes a digital television signal being demodulated to generate a baseband symbol code signal accompanied at times by artifacts of interference from a co-channel analog television signal. A cascade filter connection is included in the receiver for supplying a cascade filter response to the digitized demodulator response. An adaptive channel equalization filter provided with adjustable filtering coefficients is included in the cascade filter connection together with a comb filter for suppressing the demodulation artifacts of interference from a co-channel analog television signal. The receiver includes a symbol decoder for supplying data responsive to the cascade filter response and an intersymbol-interference suppression filter for processing the data to compensate for the intersymbol interference introduced by the comb filter. The receiver includes apparatus for extracting a received training signal from the cascade filter response during times data field synchronizing signals occur in the digital television signals. A computer is included in the receiver and calculates the terms of a discrete Fourier transform of that training signal. The computer generates a discrete Fourier transform characterizing the channel by dividing those terms by corresponding terms of a discrete Fourier transform of a comb-filtered and match-filtered response to ghost-free training signal as stored in memory for the computer. The computer calculates the adjustable filtering coefficients of the adaptive channel equalization filter so as to complement the channel characterization.
U.S. patent application Ser. No. 09/151,209 further describes a receiver including a detector for determining whether or not there is significant interference from a co-channel analog television signal. When there is not significant interference from a co-channel analog television signal, the computer still calculates the terms of a discrete Fourier transform of that training signal. The computer generates a discrete Fourier transform characterizing the channel by dividing those terms, however, by corresponding terms of a discrete Fourier transform of a match-filtered, but not comb-filtered, response to ghost-free training signal as stored in memory for the computer. The computer calculates the adjustable filtering coefficients of the adaptive channel equalization filter so as to complement the channel characterization.
The differential delays involved in the comb filters described in U.S. patent application Ser. No. 09/151,209 are for the most part considerably shorter than the 48.4 millisecond duration of a data field. The exception is a comb filter having two-NTSC-frame differential delay. The longer-differential-delay comb filters described in U.S. patent application Ser. No. 09/151,209 combine NTSC signal from scan lines that are either correlated or anti-correlated in terms of image content so the comb filter will suppress the demodulation artifacts of the NTSC co-channel interference. U.S. patent application Ser. No. 09/151,209 describes the accumulation of training signal extracted over a prescribed even number 2N of successive data fields to generate a received ghost-cancellation reference (GCR) signal with attendant ghosts.
There is no indication in U.S. patent application Ser. No. 09/151,209 that the number 2N of successive data fields over which accumulation of training signal takes place is a parameter that affects results beyond the reduction of random noise one skilled in the art would expect from the teaching in U.S. Pat. No. 5,331,416. There is no indication of there being preferred values of 2N for reducing the deleterious effects of NTSC co-channel interference.
The phase of each of the demodulation artifacts of co-channel NTSC video carrier, chroma subcarrier and sound carrier changes by a certain increment from one data field synchronization (DFS) signal to the next one data field later. Accumulation of training signal from successive DFS signals is preferably done such that the various phases of each these demodulation artifacts accumulates in accordance with vector algebra to low value while the DFS signal components accumulate in accordance with scalar alegebra to high value.
The filter coefficients of an adaptive equalizer for a digital television receiver are adjusted responsive to baseband symbol coding from which artifacts of NTSC co-channel interference have been suppressed by filtering. In accordance with the invention the filter coefficients of an adaptive equalizer for a digital television receiver are adjusted by a training-signal method using a training signal extracted from baseband symbol coding in which artifacts of NTSC co-channel interference have been suppressed in the following way. The data field synchronizing signals of a cycle of six data fields or a plurality of such cycles are accumulated. In this accumulation procedure the data field synchronizing signals from initial data fields of each data frame are accumulated in one sense of polarity, and the data field synchronizing signals from final fields of each data frame are accumulated in the opposite sense of polarity. The artifacts of co-channel NTSC interference are suppressed in the accumulation result, from which the training signal is extracted.